The present invention relates to a method for controlling the distribution of the pressure load applied to a web passing through a nip formed between a pressure zone-adjustable device, such as a variable-crown roll and/or a press-shoe apparatus, and its counter-member, such as a counter-roll, in a direction transverse to the running direction of the web, in which method loading elements acting upon the mantle or shell of a pressure zone-adjustable roll or the band of a press-shoe apparatus, such as glide-shoe groups supported against the central axle of the zone-adjustable roll, are used. The pressure generating actuator of the loading elements is controlled by means of a regulating unit.
The present invention also relates to apparatus for treating a web, such as a paper web, in a nip, such as a dewatering nip or a calendering nip, the apparatus including a zone-adjustable device, such as a variable-crown roll or a corresponding shoe device, and a counter-member, such as a counter-roll, which together form a nip through which the web to be treated is passed. The variable-crown roll or press-shoe device comprises a stationary part and a cylindrical mantle or band rotating around the stationary part, and a series of glide shoes or the like arranged between the stationary part and the mantle or band and grouped in pressure loading zones, each of which is loaded by means of zone pressures controlled by valves or the like. The apparatus further includes a regulating system which comprises a set value component, a limiter unit or corresponding processing unit, a regulator unit and an actuator unit which includes a series of pressure valves and a series of pressure-current converters or the like from which feedback signals are passed to the regulator unit.
In paper machines and after-treatment apparatus for paper, rolls are used to form dewatering press nips, smoothing nips or calendering nips in conjunction with counter-rolls. In such nips, it is important that the distribution of the linear load, i.e., the load profile, in the axial direction of the roll remains constant or that this profile can be adjusted as desired such, for example, in order to control the moisture profile and/or the thickness and/or the profile of any other corresponding property of the web, in the transverse direction of the web. For such purposes, various adjustable crown or variable-crown rolls are known by means of which the linear load distribution in a nip can be controlled.
Several different variable-crown or adjustable-crown rolls for paper machines are known. Generally, such rolls comprise a massive, stationary roll axle and a roll mantle rotatably mounted over the axle. Between the axle and mantle, glide-shoe arrangements and/or pressure-fluid chambers are arranged which act upon the inner surface of the mantle and which are divided or grouped into several parts or groups or zones in the axial direction of the roll so that the axial profile of the mantle at the nip can be aligned or adjusted as desired. As a rule, the nips formed by such rolls, such as press nips or calendering nips, are loaded by means of loading forces applied to the axle journals of the variable-crown roll and of its counter-roll.
An example of a variable-crown roll to which a method and apparatus in accordance with the invention can be advantageously applied is disclosed in Finnish Patent Application No. 864564, corresponding to pending U.S. application Ser. No. 034,167, now allowed, U.S. Pat. No. 4,757,585, owned by the assignee of the instant application.
As is known in the prior art, glide shoes loaded by means of cylinders provided with common hydraulic supply zones are used for controlling the deflection of variable-crown rolls. Each of the zones is controlled by means of a hydraulic valve which is specific to that zone. The number of glide shoes in different zones may be different from zone to zone as determined by the manner in which the compression force between the variable-crown roll and its counter-roll is to be controlled. Generally, one loading cylinder is provided at each end of the roll axle to produce the nip pressure together with the glide shoes.
It will be understood that as used herein, the adjustable zones of a pressure zone-adjustable device includes the loading members or groups of loading members extending axially along the length of the device as well as the loading members, if any, that load the ends of the device and produce the nip pressure.
Variable-crown rolls have found increasing use both in paper machines as well as in paper finishing machines and various after-treatment devices for paper. Such increased use is partly due to the fact that ever higher quality requirements are being imposed on paper products, i.e., various properties of the paper must be within ever stricter quality specifications both in the machine direction as well as in the transverse direction. At least one reason for the stricter quality standards is the advent of new copying and printing techniques which require extremely uniform paper quality in order to operate on a continuous basis. Pressure zone-adjustable devices, such as variable-crown rolls, can be used to positively affect various quality properties of paper.
Although the mechanical constructions of variable-crown rolls have been considerably developed in recent years, the same cannot be said about systems for regulating the variable-crown rolls. However, such regulating systems are very important where variable-crown rolls are used to control the quality properties of paper.
Conventional control or regulating systems for pressure zone-adjustable devices have the drawback that, even if the interaction of the compression forces produced by the different zone pressures in different zones of the zone-adjustable devices are taken into account, the operator only has a relatively small number of zones available for control in any attempt to regulate or control the profile of web properties in the transverse direction. An example of such a control system in use at present is disclosed in German Patent DE No. 3,117,516 corresponding to U.S. Pat. No. 4,464,921.
For example, considering an embodiment in which a variable-crown roll includes five pressure zones, it is possible using conventional regulating systems to set the linear load at five different points in accordance with five respective set values. If the length of the variable crown roll is, for example, ten meters, the points at which the linear load can be set are located about two meters apart from each other and it is not possible to control with any degree of accuracy the linear load acting in the areas between the points at which the linear loads are actually set. An increase in the number of actual pressure zones in the pressure zone adjustable-device results in a more complicated construction of the device, e.g., the variable-crown roll, and a greater possibility of malfunctioning so that this possbility is not a solution.